Chemical sensors have been developed for decades to detect various concentration levels of gases and vapors for deployment in a wide range of applications in industry, space mission, environment monitoring, medical, military, and others. The detection usually centers on change in a particular property or status of the sensing material (such as thermal, electrical, optical, mechanical, etc) upon exposure to the species of interest. The sensing material may be any of several elements from the periodic table, plus inorganic, semiconducting and organic compounds, in bulk or thin film form. One of the most investigated classes of chemical sensors in the past is the high-temperature metal oxide sensors due to these sensors' high sensitivity at low ppm to ppb concentration levels, with tin oxide thin films as an example. Polymer sensors have been studied in recent years because they can be operated at room temperature with low power consumption and are easily fabricated. Although commercial sensors based on these materials are available, continued research is in progress with sensing technologies using new sensing materials and new transducer platforms. New sensing technologies, such as nanotechnology-based sensors, are being developed to overcome the large power consumption and poor selectivity of metal oxides sensors, and to improve the poor sensitivity and narrow detection spectrum of polymer sensors.
Typical figures of merit expected from a chemical sensor include sensitivity (even down to a few molecules, selectivity, low power consumption, rapid response time and rapid sensor recovery time. Sensors based on the emerging nanotechnology may provide improved performance on all of the above aspects compared to current micro and macro sensors. Nanomaterials exhibit small size, light weight, very high surface-to-volume ratio, and increased chemical reactivity compared to bulk materials; all these properties are ideal for developing extremely sensitive detectors. The potential of nanomaterials for detection and for protection and remediation has been outlined in a recent report by the National Science and Technology Council (“Nanotechnology Innovation for Chemical, Biological, Radiological and Explosive Detection and Protection,” November 2002).
One promising nanomaterial is the carbon nanotube (“CNT”), which exhibits extraordinary mechanical, electrical and optical properties. These interesting properties have prompted wide range investigations for applications in nanoelectronics, high strength composites, field emitting devices, catalysts, etc. A single-walled carbon nanotube (“SWCNT”) has all the atoms on the surface and therefore would be exposed maximally to the environment, allowing a change in its properties sensitively. The first demonstration of an SWCNT-based sensor was for a chemical field effect transistor (“CHEMFET”), where a single semiconducting SWCNT is used as the channel material and the conductivity was shown to change upon exposure to NO2 and NH3 (J. Kong et al, “Nanotube Molecular Wires as Chemical Sensors,” Science, vol. 287, pp 622-625, January 2000). The potential for using CNTs in chemical sensors was noted in the Kong et al article. However, it still is a challenge to make practical sensors, due to difficulty in fabrication complexity, low sensor yield and poor reproducibility.
A different configuration of carbon nanotube-based chemical sensors with much easier fabrication process was introduced by J. Li et al in “Chemical and Physical Sensors, Carbon Nanotubes: Science and Applications”, M. Meyyappan, ed., CRC Press, Boca Raton, Fla., 2004. First, an interdigitated electrode (“IDE”) configuration is fabricated using conventional photolithographic methods with a nominal finger width of 10 μm and gap size of 8 μm. The electrode fingers are made of thermally evaporated Ti and Au (20 nm and 40 nm thickness, respectively) on a layer of SiO2, thermally grown on top of a silicon wafer. Second, a thin layer of carbon nanotubes forming a network is laid on the fingers using a solution casting process. The conductivity of the CNT network changes upon exposure of the fingers to different gases or vapors. Such a process is significantly simpler and produces consistent sensors based on statistical properties of the CNT network with high yield (˜100 percent). The IDE configuration facilitates effective electric contact between SWCNTs and the electrodes over large areas, while providing good accessibility for analytes in the form of gas/vapor adsorption or contaminants adsorption/extraction in/from liquid to all SWCNTs including semiconducting tubes.
Although carbon nanotube-based chemical sensors using the IDE configuration have been prototyped in laboratories, use of an IDE configuration for wireless sensing has not been reported in published research. Use of an IDE configuration requires frequent transmission of data measured at each of a large number of IDE fingers. A straightforward approach may consume substantial power and may require use of a relatively large footprint for the sensor system. This may be inconsistent with automated use of a remote system in a confined space.
What is needed is a wireless transmission system that (1) has a small footprint and whose size is compatible with a nanosensor system, which may have a diameter as small as 5-15 cm, (2) consumes relatively little power (e.g., about 50 μWatt-60 mWatt, or smaller with voltage regulation not activated), (3) is reliable, with a mean time to failure exceeding 8 hours (representative battery life) and (4) permits frequent data transmission from each of a large number of data sources.